Conventional Ziegler-Natta catalysts consist of a main catalyst component (also known as pro-catalyst) and a cocatalyst component. The main catalyst component comprises typically a compound of a transition metal from Group IVB of the Periodic Table, in particular a titanium compound. The cocatalyst component is typically an organoaluminium compound, for example, Al(C2H5)3, Al(C2H5)2Cl, Al2(C2H5)3Cl3, and the like. Since coming out in 1950's, Ziegler-Natta type catalysts have developed over several generations and have properties greatly improved. For example, the development of supported catalysts has markedly enhanced the polymerization activities of Ziegler-Natta catalysts.
Active magnesium dichloride is a most effective carrier of Ziegler-Natta catalysts. There exist a number of known techniques that utilize active magnesium dichloride carrier or active magnesium dichloride-inorganic oxide complex carrier to prepare Ziegler-Natta catalysts. Many magnesium compounds, such as commercial available anhydrous magnesium dichloride, dialkyl magnesium compounds, dialkoxy magnesium compounds, Grignard reagent, magnesium compounds in nascent state (see, for example, CN1041312C), and the like, have been used as sources of active magnesium dichloride in the prior art. These different methods give generally catalysts different from each other in chemical composition, distribution of active sites, polymerization behavior, and property of the obtained polymers.
Most of known techniques utilize a single magnesium compound as a source of active magnesium dichloride in the preparation of a Ziegler-Natta catalyst. There are also techniques that utilize two or more magnesium compounds as sources of active magnesium dichloride in the preparation of a Ziegler-Natta catalyst.
For example, CN 1051318C utilizes both a magnesium halide and an alkoxy magnesium to prepare a pro-catalyst in a process comprising contacting a particulate inorganic support with a chlorinating agent, and then impregnating the particulate inorganic support with an impregnating solution comprising i) a magnesium dihalide, ii) an alkoxy magnesium, iii) an alkoxy titanium compound, and iv) an electron donor.
CN 1299375A discloses a catalyst component for olefin polymerization prepared by contacting a MgCl2 or ROMgCl-coated carrier with an alkyl metal halide, then with a magnesium-containing composition R2n2(R3O)2-n2Mg, and then with a titanium halide.
CN 1072683C discloses a process for preparing a catalyst, comprising (i) activating a silica support by contacting it with a solution of a dialkyl magnesium or an alkyl magnesium chloride in a liquid aliphatic hydrocarbon solvent, (ii) impregnating the activated silica support with a solution of TiCl4 and a tetraalkoxy titanium as well as MgCl2 in a liquid aliphatic or aromatic ester, and (iii) treating the solid from step (ii) with an alkyl aluminium sesquichloride.
U.S. Pat. No. 6,365,540 B1 discloses a compound containing magnesium, halide, and alkoxy and having a formula MgpXq(OR)2p-q, preferably MgCl2[Mg(OR)2]2. This compound is formed by reacting MgCl2 with an alcohol ROH in benzene, and then with an alkyl magnesium R′2Mg. Impregnating a reaction product of this compound with TiCl4 on a carrier will give a catalyst.
These patents and patent application use two kinds of magnesium compounds during the catalyst preparation, and convert said magnesium compounds into active magnesium dichloride by different methods. Their purposes are to find a catalyst exhibiting a good balance in activity and hydrogen response, or to improve gelation property of polymers, or to enhance activities of catalysts. There is not prior art that teaches or suggests the use of two or more magnesium compounds in the preparation of a main catalyst component for olefin polymerization in order to broaden a molecular weight distribution of the polymer obtained by using said main catalyst component.
It is known in the art that a polyethylene having broader molecular weight distribution will have better processibility, and many researches focus on the preparation of polyethylenes having broad or bimodel molecular weight distribution through a multi-stage polymerization process or by using a complexed catalyst system.
Therefore, it is desired to develop a catalyst which is capable of producing a polyethylene having broader molecular weight distribution in a single polymerization stage in one reactor.